[llvm-bugs] [Bug 42037] New: C++2a std::midpoint's "Constraints" are not implemented

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42037

            Bug ID: 42037
           Summary: C++2a std::midpoint's "Constraints" are not
                    implemented
           Product: libc++
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

// https://godbolt.org/z/mAL4Sd
#include <numeric>
int a();
int b();
int main() {
    std::midpoint(a, b);
}

====

/c++/v1/numeric:554:52: error: arithmetic on pointers to the function type 'int
()'
    return __a + _VSTD::midpoint(ptrdiff_t(0), __b - __a);
                                               ~~~ ^ ~~~

According to N4810, std::midpoint for `T*` should be constrained to require `T`
to be a complete object type (not a function type, and not cv void either).

I'm fairly confident that no library code should ever try to constrain on
`is_pointer` without also asking whether it's an object pointer, function
pointer, member pointer, or void pointer. It might be worth auditing the entire
libc++ codebase for uses of `is_pointer`.

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